1. Field of the Invention
The present invention relates to a high-frequency signal transmission line and more particularly relates to a high-frequency signal transmission line including a ground conductor and a signal line.
2. Description of the Related Art
In a high-frequency signal transmission line having a stripline structure in which a signal line is sandwiched between ground conductors from above and below, due to the reason described below, the line width of the signal line is increased in order to decrease a high-frequency resistance value of the signal line. More specifically, when a high-frequency signal flows through the signal line, the high-frequency signal flows in a concentrated manner through the vicinity of the surface of the signal line due to a skin effect. In addition, when the high-frequency signal flows through the signal line, a high-frequency signal flows also through the ground conductors in a direction opposite to that of the high-frequency signal due to electromagnetic induction. The high-frequency resistance value of such a high-frequency signal decreases, if the surface area of the signal line and the area of a portion of each ground conductor which the signal line faces increase and conductor loss in the signal line and each ground conductor decreases. Therefore, in the high-frequency signal transmission line, the line width of the signal line is increased in order to decrease the high-frequency resistance value of the signal line.
However, when the line width of the signal line is increased, the area of a portion where the signal line and each ground conductor face each other increases, and a capacitance occurring between the signal line and the ground conductor increases. Thus, in order to set the high-frequency signal transmission line so as to have a predetermined impedance, the distance between the signal line and each ground conductor is increased and the capacitance is decreased. However, when the distance between the signal line and each ground conductor is increased, the thickness of the high-frequency signal transmission line increases and it becomes difficult to bend and use the high-frequency signal transmission line.
Thus, it is considered that the signal line and each ground conductor are not arranged to face each other. Hereinafter, this will be described in more detail with reference to the drawing. FIG. 18 is a diagram of a high-frequency signal transmission line 500 in which a signal line 502 is exposed from a ground conductor 504, as seen in a planar view from a lamination direction.
As shown in FIG. 18, the high-frequency signal transmission line 500 includes the signal line 502 and the ground conductors 504 and 506. The signal line 502 is a line-shaped conductor. The ground conductor 506 is provided on the lower side of the signal line 502 in the lamination direction and faces the signal line 502 through a dielectric layer. The ground conductor 504 is provided on the upper side of the signal line in the lamination direction and has an opening. The signal line 502 is located in the opening when being seen in a planar view from the upper side in the lamination direction.
In the high-frequency signal transmission line 500 shown in FIG. 18, the signal line 502 and the ground conductor 504 do not overlap each other when being seen in a planar view from the lamination direction. Thu, a capacitance occurring between the signal line 502 and the ground conductor 504 in the high-frequency signal transmission line 500 is smaller than a capacitance occurring between a signal line and a ground conductor in a high-frequency signal transmission line in which the signal line and the ground conductor overlap each other. Due to this, in the high-frequency signal transmission line 500, it is possible to decrease the distance between the signal line 502 and the ground conductor 504. As a result, in the high-frequency signal transmission line 500, the thickness of the high-frequency signal transmission line 500 can be decreased, and it becomes possible to bend and use the high-frequency signal transmission line 500.
However, the high-frequency signal transmission line 500 has a problem that unwanted radiation from the signal line 502 occurs. The signal line 502 does not overlap the ground conductor 504. Thus, an electromagnetic field generated by a current flowing through the signal line 502 is not absorbed by the ground conductor 504 and is radiated through the opening to the outside of the high-frequency signal transmission line 500, and unwanted radiation occurs.
As a high-frequency signal transmission line that can solve the above problem, for example, a flexible board described in Japanese Unexamined Patent Application Publication No. 2007-123740 is known. FIG. 19 is a diagram of a flexible board 600 described in Japanese Unexamined Patent Application Publication No. 2007-123740, as seen in a planar view from a lamination direction.
The flexible board 600 includes a signal line 602 and a ground layer 604. The signal line 602 is a line-shaped conductor. The ground layer 604 is laminated on the upper side of the signal line 602 in the lamination direction via a dielectric layer. In addition, although not shown, a ground layer is provided on the lower side of the signal line 602 in the lamination direction. In the flexible board 600, a plurality of openings 606 is provided in the ground layer 604. The openings 606 have rectangular shapes and are arranged on the signal line 602 in a line. Thus, the signal line 602 partially overlaps the ground layer 604 when seen in a planar view from the upper side in the lamination direction. As a result, in the flexible board 600, since the signal line 602 overlaps a portion of the ground conductor 604 where no opening is formed, unwanted radiation from the signal line 602 is reduced in this portion.
However, in the flexible board 600 described in Japanese Unexamined Patent Application Publication No. 2007-123740 as well, unwanted radiation occurs through the openings 606.